Many polymeric materials are hydrophobic. This property can restrict the use of the materials in applications in which they come into contact with water or aqueous solutions because articles made from such hydrophobic materials are not wetted. Similarly, a polymeric material which is hydrophilic might be unsuitable for an application in which hydrophobic properties are required.
Techniques are known for the treatment of polymeric materials which are hydrophobic so that they can be wetted by aqueous solutions. For example, International Application No. WO-93/01622 discloses a technique in which a non-woven fabric made from a polyolefin such as polypropylene is exposed to ultraviolet radiation while impregnated with a solution of acrylic acid. This results in a grafting reaction between the acrylic acid and the polypropylene of the fiber surfaces, and in crosslinking of the polypropylene. The resulting fabric can be wetted by aqueous solutions. Furthermore, the wettability does not degrade on exposure to alkali, and the tensile strength of the fabric is not adversely affected by the grafting treatment. The fabric which results from this treatment can be used as an electrode separator in an electrochemical device in which the electrolyte comprises an aqueous solution.
The properties of a fabric which is produced by the technique disclosed in WO-93/01622 depend on the amount of the vinyl monomer which is grafted to the fabric. It is important that sufficient monomer is grafted to the fabric for the fabric to be wetted. However, vinyl monomer which is grafted to the fabric can tend to swell when the fabric is exposed to an aqueous solution. This is undesirable because it can reduce the porosity of the fabric. Accordingly, while the amount of vinyl monomer which grafts to surface of the fabric should be sufficient for the fabric to be wetted, it should be controlled and not allowed to reach unnecessarily high levels.